In an interview with Rolling Stone magazine, Natalie Portman once said: "I would never have been an actress if I weren't an only child, because my parents would never have let me be the star of the family at the expense of another child." It turns out that when it comes to the advantages of life without brothers and sisters, she was on to something.

One of the widest-ranging research projects on family life conducted in Britain has revealed that the fewer siblings children have, the happier they are – and that only children are the most contented.

The findings, shared exclusively with the Observer, suggest that "sibling bullying" could be part of the problem, with 31% of children saying they are hit, kicked or pushed by a brother or sister "quite a lot" or "a lot". Others complain of belongings being stolen by siblings and being called hurtful names.

The figures are the first to emerge from Understanding Society, a study tracking the lives of 100,000 people in 40,000 British households. They will be revealed on Friday in Britain in 2011, the State of the Nation, a magazine published by the Economic and Social Research Council.

On children and happiness, it finds that:

• Seven out of 10 British teenagers are "very satisfied" with their lives.

•Children from ethnic minorities are on average happier than their white British counterparts.

• Happiness declines the more siblings there are in a household.

The findings are based on in-depth questionnaires completed by 2,500 young people, which have been analysed by Gundi Knies from the Institute for Social and Economic Research at the University of Essex where the Understanding Society study is based. She suggested that factors such as competition for the parents' attention or the fact that toys, sweets or space need to be shared could be to blame. Knies also pointed to other data within the study on sibling bullying: 29.5% of teenagers complain of being called "nasty names" by brothers or sisters "quite a lot" or "a lot", while 17.6% say they have their belongings taken away from them.

Professor Dieter Wolke of the University of Warwick, who carried out the work on tensions between brothers and sisters, said: "More than half of all siblings (54%) were involved in bullying in one form or the other." Although there is also evidence that points to siblings providing support for each other, he warned that children who faced bullying both at home and in the playground were particularly vulnerable to behaviour problems and unhappiness.

Wolke did not study the impact of such tensions on parents, but added: "From anecdotal reports, quarrelling siblings increase stress for parents and some just give up intervening or intervene inconsistently, leaving the field wide open for the bully sibling."

Siobhan Freegard, the co-founder of the website Netmums who has three children, said that many mothers felt like "referees" after their children reached a certain age and started quarrelling with their brothers and sisters.

She questioned whether the findings on happiness were linked to the fact that children were desperate for parental attention. "With three children, it is three lots of dinner, three lots of washing, three lots of driving to after-school activities, so you do get less time for each. I like to think they are getting benefits in other ways," said Freegard.

She said the findings would come as a relief to the parents of only children who often felt guilty about the lack of brothers and sisters.

Freegard discussed the issue recently with her friend, Tanya Honey, who has one child, a daughter. Honey admitted that her daughter, Gemma, seven, recently wrote "a baby" on her shopping list. "But friends always point out that she is a really happy child. When we go on holiday she is brilliant at making friends and if there was a brother or sister perhaps she wouldn't be, because she would rely on them," added Honey.

While the findings seem surprising, experts say there are clear reasons why more siblings could reduce happiness. Dr Ruth Coppard, a child psychologist, said: "In an average home the more children, the less privacy for each child. Some love sharing a bedroom with a sibling but they would rather choose to do it than have to do it. There is competition for parental time."

Coppard said she made a conscious decision to have just two children because more would become unaffordable. "After that I would need a bigger car, more bedrooms, holidays would be difficult," she said. But she argued that there were also issues for only children, who were the "sole recipients of parental expectations".

Parentline Plus, a charity that offers support to parents, regularly receives calls about sibling rivalry. "Families do report concerns regarding high levels of conflict among siblings and the stress that this can cause, but the important thing is to try to help and support families find more effective ways of dealing with this problem," said Alison Phillips, director of policy and communications.

She has issued a series of tips to parents including: don't be too quick to blame, even if one child looks innocent; ensure children have a special place for their belongings; insist they ask if they want to use something owned by a sibling, and show firmly that you do not approve of bullying behaviour.

Twelve years younger than her only brother, Kelly Rose Bradford grew up feeling like an only child – lonely and used to her own company. Now 40 and a single mother, she feels guilty that her son has no siblings to share his life with